Fixes #36999 - Drop libvirt CPU model option for VM creation

[ekohl]

Since fog-libvirt 0.12.0 the default CPU model was some abstract type with a low compatibility level. Since 0.12.0 it defaults to CPU passthrough, as recommended by qemu if live migration is not required (https://www.qemu.org/docs/master/system/qemu-cpu-models.html). That's not something we support today, so it should be safe to drop the CPU model selection.

RFC open for this: https://community.theforeman.org/t/drop-cpu-type-option-from-libvirt-compute-resource-provisioning/36267